Radar polarization calibration and correction

• US 7,345,625 B1
• Filed: 09/28/2005
• Issued: 03/18/2008
• Est. Priority Date: 09/28/2005
• Status: Active Grant

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Urkowitz, Harry
• Primary Examiner(s): Sotomayor; John B
• Application Number:
  US11/237,531
• Time in Patent Office: 902 Days
• Field of Search: 342159-162, 342/173, 342/174, 342/188, 342361-366
• US Class Current: 342/174
• CPC Class Codes: G01S 7/026 involving the transmission of elliptically or circularly polarised waves
• CPC Class Codes: G01S 7/4017 of HF systems

Abstract

A polarimetric radar system transmits a signal which is nominally the desired polarization, but which may deviate therefrom. A calibration operation is performed using a symmetrical radar reflector, to determine the gains and phases of reception of two mutually orthogonal components of the received reflected signal which result in nulling of the two polarization components of the reflected signal. These gains and phases represent receive corrections which result in a simulation of perfect polarization purity on both transmission and reception. The corrections established during calibration are applied to the receive signals during normal (non-calibration) operation, to improve the effective polarization of the transmission and reception.

Claims

1. A method for compensating errors in transmitted circular polarization such that reflections from spherical objects tend to be cancelled, said method comprising:

transmitting from an antenna an electromagnetic signal at a carrier frequency, which signal includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular elliptical polarization, to thereby produce transmitted signals;

receiving, by means of first and second linear antennas, first and second mutually orthogonal linear components of reflections of said transmitted signals, to thereby produce first and second received signals;

procuring a set of correction factors to the first and second received signals, which correction factors are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar;

processing said first and second received signals with said correction factors in such a fashion as to cause the received signal to appear as if substantially perfect circular polarization had been transmitted and received.

2. A method according to claim 1, wherein said step of procuring a set of correction factors comprises the steps of:

placing a sphere to receive said transmitted signal and reflect said reflected signal at a plurality of beam directions;

adjusting the amplitudes and phases of said first and second received signals to cancel the combined echoes at each of said beam directions; and

tabulating said amplitudes and phases at which said cancellation occurs for each beam direction to generate said set of correction factors.

3. A method for determining the ellipticity of a radar target, said method comprising the steps of:

transmitting from an antenna an electromagnetic signal at a carrier frequency, which signal includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular elliptical polarization, to thereby produce transmitted signals;

receiving, by means of first and second linear antennas, first and second mutually orthogonal linear components of reflections of said transmitted signals, to thereby produce first and second received signals;

procuring a set of correction factors to the first and second received signals, which correction factors are a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar;

processing said first and second received signals with said correction factors in such a fashion as to cause the received signal to appear as if substantially perfect circular polarization had been transmitted and received; and

receiving from a nonspherical target third and fourth mutually orthogonal linear components of reflections of said transmitted signals;

processing said third and fourth received signals with said correction factors to thereby generate corrected mutually orthogonal received components; and

comparing the characteristics of said corrected mutually orthogonal received components to determine the nonsphericity of said target.

4. A method for compensating errors in transmitted and received circular polarization, said method comprising:

transmitting from an antenna an electromagnetic signal at a carrier frequency, which signal includes mutually orthogonal linear components nominally phased to generate circular polarization, but which may instead generate noncircular elliptical polarization, to thereby produce transmitted signals;

receiving by means of first and second linear antennas first and second mutually orthogonal linear components of reflections of said transmitted signals, to thereby produce first and second received signals;

procuring a set of correction factors to the first and second received signals, which correction factors are a function of carrier frequency, and a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar;

processing said first and second received signals with said correction factors in such a fashion as to cause the received signal to appear as if substantially perfect circular polarization had been transmitted and received.

5. A method for compensating errors in transmitted andor received linear polarization, said method comprising the steps of:

transmitting from an antenna an electromagnetic signal at a carrier frequency, which signal includes two mutually orthogonal linear components nominally controlled in phase and amplitude to produce linear polarization, but which may be at a phase or amplitude such that (a) the polarization of the transmitted signal is at an undesired angle and (b) the transmitted signal includes a circular component, to thereby produce first and second transmitted signals;

receiving by means of first and second linear antennas first and second mutually orthogonal linear components of reflections of said transmitted signals, to thereby produce first and second received signals;

procuring a set of correction factors to the first and second received signals, which correction factors are a function of carrier frequency, and a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar;

processing said first and second received signals with said correction factors in such a fashion as to cause the received signal to appear as if (a) substantially perfect linear polarization had been transmitted and received in the desired direction.

6. A method according to claim 5, wherein said step of procuring a set of correction factors comprises the steps of:

placing a sphere to receive said transmitted signal and reflect said reflected signal at a plurality of beam directions;

adjusting the amplitudes and phases of said first and second received signals to cancel the combined echoes at each of said beam directions; and

tabulating said amplitudes and phases at which said cancellation occurs for each beam direction to generate said set of correction factors.

7. A method for determining the nonsphericity of a radar target, said method comprising the steps of:

transmitting from an antenna an electromagnetic signal at a carrier frequency, which signal includes two mutually orthogonal linear components nominally phased to produce linear polarization, but which may be at a phase which such that (a) the polarization of the transmitted signal is at an undesired angle and (b) the transmitted signal includes a circular component, to thereby produce first and second transmitted signals;

receiving by means of first and second linear antennas first and second mutually orthogonal linear components of reflections of said transmitted signals, to thereby produce first and second received signals;

procuring a set of correction factors to the first and second received signals, which correction factors are a function of carrier frequency, and a function of (a) beam direction in the case of monostatic radar and (b) beam directions in the case of bistatic radar;

processing said first and second received signals with said correction factors in such a fashion as to cause the received signal to appear as if (a) substantially perfect linear polarization had been transmitted and received in the desired direction;

placing said radar target for reflection of said first and second transmitted signals, to thereby produce third and fourth received signals;

applying said correction factors to said third and fourth received signals, to thereby produce corrected third and fourth received signals; and

comparing said corrected third and fourth received signals to determine the nonsphericity of said radar target.